Many electronic circuits require a stable and accurate reference voltage for effective operation. Reference voltages, however, may be unstable due to temperature variations caused during circuit operation. To compensate for the temperature dependence of reference voltages, bandgap circuits have been designed to minimize the effect of temperature on the reference voltage. These conventional bandgap circuits compensate for the first order temperature coefficient of a transistor's base to emitter voltage without completely eliminating the temperature dependent characteristics of the circuit. Thus, the base to emitter voltage remains dependent on changing operating and process characteristics.
FIG. 1a illustrates a typical bandgap circuit 10. The current source 12 is designed to increase with temperature using the same type of resistivity as resistor 14. In other words, as the temperature goes up, the current will also go up and, as a result, the voltage across resistor 14 will go up. The diode 16, on the other hand, has a negative temperature coefficient. In this case, as the temperature goes up, the voltage across diode 16 will go down. With proper trimming, the circuit 10 can be designed to provide a constant, to the first order, bandgap voltage V.sub.BG across both resistor 14 and diode 16.
As illustrated in FIG. 1b, the bandgap voltage V.sub.BG as function of temperature will not be constant to higher orders. In typical applications, the circuit will be tuned such that it has a zero temperature coefficient at some predetermined temperature T.sub.0, typically room temperature (e.g., 25.degree. C.). In some applications, this variation creates issues and therefore it is desirable to correct the higher order effects.
Most techniques used in the past to correct the curvature of the bandgap reference usually vary too much with process and introduce extra errors which are not trimmed out. These techniques limit the performance of the bandgap reference at the best of .+-.1% specification over the full military (e.g., -50.degree. C. to 150.degree. C.) temperature range.